Use of pulse induction logging while drilling (“LWD”) resistivity measurements in downhole environments provides information about formations surrounding the borehole. Use of such techniques allows the continuation of drilling while acquiring information needed for drill steering, or to determine proximity to formation interfaces, such as gas-oil, gas-water, or water-oil interfaces.
U.S. Pat. No. 7,167,006 (“the '006 patent”) to Itskovich, the specification of which is incorporated herein by reference, describes an apparatus and method for a pulse induction LWD system using a multi-receiver array. Use of that invention provides improved resolution of signals, allowing resolution of signals that would otherwise be unresolvable. This improved resolution is accomplished in that case by acquiring a calibration signal while the measurement tool is outside of the formation, and subtracting the calibration signal from the measurement signal obtained while the tool is in the downhole environment.
While the calibration technique of the '006 patent provides improved resolution, still further improvements in pulse induction LWD measurements are possible. Use of two receivers in the tool can allow time-dependent calibration signals to be acquired from both receivers. These calibration signals can then be combined to create a time-dependent calibration coefficient. When pulse induction LWD measurements are taken downhole, the measurement signals received by the two receivers can be combined with the calibration coefficient to generate a time-dependent differential measurement signal. This high resolution signal provides an improved ability to resolve interfaces in the formation surrounding the borehole.
Accordingly, it is an object of the invention to provide improved resolution of boundary locations in formations surrounding boreholes.
It is another object of the invention to provide measurements of boundaries in formations for use in real-time geo-steering of drilling operations.
It is yet another object of the invention to provide measurements to determine the location of interfaces in a formation, such as gas-water, gas-oil, or water-oil interfaces.